A femto base station refers to a micro mobile communication base station that is connected to a mobile communication core network via a broadband network installed indoors, for example a home or an office. The term ‘femto-’ is a prefix denoting a factor of 10−15.
A femto base station refers to a base station that covers a radius equal to or less than 10 m. A femto base station is advantageous in that it can extend an indoor coverage, improve call quality, and efficiently provide various types of wired and wireless convergence services. A femto base station has a relatively small cell coverage, compared with the macro base station, and can be installed to places that the closed subscriber group (CSG) desires.
In an environment where only conventional macro base stations are installed, a mobile station can easily identify a base station that it intends to access. However, in an environment where femto base stations may be installed with 5000˜7000/km2, it is not easy for a mobile station to identify whether an adjacent base station, which it intends to access, is a macro base station or a femto base station.
In recent years, femto base stations have been operated in a closed access mode where they provide services to only closed subscriber group that are registered to themselves or in an open access mode where they provides services to groups that are not registered to themselves. Femto base stations can also be operated in a semi open access mode that has the features of both the closed access mode and open access mode.
A closed subscriber group (CSG) in 3GPP, supporting a femto base station, stores a white list of femto base stations that can use a SIM or USIM of a mobile station and the unit of a cell supporting a CSG is composed of one or more femto base stations. Therefore, the CSG needs a CSG ID (29 bits) as well as a femto base station ID. However, conventional WiMAX system requires to a one-to-one mapping between a mobile station and a femto base station, without using a CSG ID. Therefore, conventional CSG in 3GPP is using a method that does not match a WiMAX system.